The present invention relates in general to sewing machines and in particular to a new and useful sewing machine which includes a mechanism for automatically making edge-parallel seams by using a piston which can be lowered onto a workpiece to either actively rotate the workpiece about an axis of the needle or passively hold the workpiece while the workpiece is moved by a feed dog acting on the bottom of the workpiece.
U.S. Pat. No. 4,526,117 to Willenbacher shows a sewing machine with a pressure piston for the edge-parallel sewing of outer scallops, which is placed on the workpiece and is controlled by a scanning device with the sewing machine running. At that point a feed dog of the machine which continues to operate, rotates the workpiece about the pressure piston serving as brake means and holds it in abutment at the relatively narrow guide ruler. For the edge-paralell sewing of inner scallops, the guide ruler alone serves to perform the rotary movement of the workpiece, in that the workpiece braces itself on the guide ruler and in so doing is pushed away laterally, whereby it executes a clockwise rotation at least in the region of the stitch formation point. With this type of sewing machine, however, only workpieces with alternately straight and arcuate edges can be worked automatically. If, on the contrary, the workpiece has angular edges, the corner-shaped seam section cannot be sewn automatically. Instead, the sewing machine must be stopped with the needle inserted in the corner point of the seam and the workpiece must then be rotated around the needle by hand. It is noted that U.S. Pat. No. 4,526,117 has the same inventor and assignee as the present application.
From U.S. Pat. No. 3,425,369 to Kosrow, a sewing system is known which serves for the automatic edge-parallel sewing of workpieces with workpiece edges meeting at an acute angle. For this purpose, the sewing system comprises a workpiece rotating device with a tappet lowerable onto the workpiece, the size of the angle of rotation being determined by a scanning device responding to the workpiece edge. The workpiece hangs down over the edge of a narrow, substantially arcuate cloth supporting plate, and due to the friction at this edge it experiences during the forward movement a rotational moment or torque by which it is pushed against a guide ruler. Before the stitch formation point, a plate provided with a spring element is arranged at a lever. When resting on the workpiece, the plate and the spring element supplement the action of the presser foot. In addition, the plate has an aligning action on the workpiece. This aligning effect presumably comes about through the fact that the plate exerts on the workpiece passing under it, a brake force which, due to the position of the plate slightly offset laterally relative to the stitch formation point, exerts on the workpiece a rotatational moment which additionally supports the rotational moment caused by the friction at the edge of the cloth supporting plate. By means of such an arrangement, whose aligning action comes about in the case of outer scallops by a relative movement between workpiece and brake means, workpieces with narrow radii can, however, be controlled only quite inaccurately.